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POLL(2)			   Linux Programmer's Manual		       POLL(2)

       poll, ppoll - wait for some event on a file descriptor

       #include <poll.h>

       int poll(struct pollfd *fds, nfds_t nfds, int timeout);

       #define _GNU_SOURCE	   /* See feature_test_macros(7) */
       #include <poll.h>

       int ppoll(struct pollfd *fds, nfds_t nfds,
	       const struct timespec *timeout_ts, const sigset_t *sigmask);

       poll()  performs a similar task to select(2): it waits for one of a set
       of file descriptors to become ready to perform I/O.

       The set of file descriptors to be monitored is  specified  in  the  fds
       argument, which is an array of structures of the following form:

	   struct pollfd {
	       int   fd;	 /* file descriptor */
	       short events;	 /* requested events */
	       short revents;	 /* returned events */

       The caller should specify the number of items in the fds array in nfds.

       The  field  fd  contains	 a  file descriptor for an open file.  If this
       field is negative, then the corresponding events field is  ignored  and
       the revents field returns zero.	(This provides an easy way of ignoring
       a file descriptor for a	single	poll()	call:  simply  negate  the  fd

       The  field  events  is  an  input  parameter, a bit mask specifying the
       events the application is interested in for the file descriptor fd.  If
       this field is specified as zero, then all events are ignored for fd and
       revents returns zero.

       The field revents is an output parameter, filled by the kernel with the
       events  that  actually  occurred.   The	bits  returned	in revents can
       include any of those specified in events, or one of the values POLLERR,
       POLLHUP,	 or POLLNVAL.  (These three bits are meaningless in the events
       field, and will be set in the revents field whenever the	 corresponding
       condition is true.)

       If  none of the events requested (and no error) has occurred for any of
       the file descriptors, then  poll()  blocks  until  one  of  the	events

       The  timeout argument specifies the minimum number of milliseconds that
       poll() will block.  (This interval will be rounded  up  to  the	system
       clock  granularity, and kernel scheduling delays mean that the blocking
       interval may overrun by a small amount.)	 Specifying a  negative	 value
       in  timeout  means  an  infinite timeout.  Specifying a timeout of zero
       causes poll() to return immediately, even if no	file  descriptors  are

       The  bits that may be set/returned in events and revents are defined in

	      POLLIN There is data to read.

		     There is urgent data to read (e.g., out-of-band  data  on
		     TCP socket; pseudoterminal master in packet mode has seen
		     state change in slave).

		     Writing now will not block.

	      POLLRDHUP (since Linux 2.6.17)
		     Stream socket peer closed connection, or shut down	 writ‐
		     ing  half	of  connection.	  The _GNU_SOURCE feature test
		     macro must be defined (before including any header files)
		     in order to obtain this definition.

		     Error condition (output only).

		     Hang up (output only).

		     Invalid request: fd not open (output only).

       When  compiling with _XOPEN_SOURCE defined, one also has the following,
       which convey no further information beyond the bits listed above:

		     Equivalent to POLLIN.

		     Priority band data	 can  be  read	(generally  unused  on

		     Equivalent to POLLOUT.

		     Priority data may be written.

       Linux also knows about, but does not use POLLMSG.

       The  relationship  between poll() and ppoll() is analogous to the rela‐
       tionship between select(2) and  pselect(2):  like  pselect(2),  ppoll()
       allows  an  application	to  safely wait until either a file descriptor
       becomes ready or until a signal is caught.

       Other than the difference in the precision of the timeout argument, the
       following ppoll() call:

	   ready = ppoll(&fds, nfds, timeout_ts, &sigmask);

       is equivalent to atomically executing the following calls:

	   sigset_t origmask;
	   int timeout;

	   timeout = (timeout_ts == NULL) ? -1 :
		     (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
	   sigprocmask(SIG_SETMASK, &sigmask, &origmask);
	   ready = poll(&fds, nfds, timeout);
	   sigprocmask(SIG_SETMASK, &origmask, NULL);

       See  the description of pselect(2) for an explanation of why ppoll() is

       If the sigmask argument is specified  as	 NULL,	then  no  signal  mask
       manipulation is performed (and thus ppoll() differs from poll() only in
       the precision of the timeout argument).

       The timeout_ts argument specifies an upper limit on the amount of  time
       that  ppoll() will block.  This argument is a pointer to a structure of
       the following form:

	   struct timespec {
	       long    tv_sec;	       /* seconds */
	       long    tv_nsec;	       /* nanoseconds */

       If timeout_ts is specified as NULL,  then  ppoll()  can	block  indefi‐

       On success, a positive number is returned; this is the number of struc‐
       tures which have nonzero revents fields (in other words, those descrip‐
       tors  with events or errors reported).  A value of 0 indicates that the
       call timed out and no file descriptors were ready.   On	error,	-1  is
       returned, and errno is set appropriately.

       EFAULT The  array  given	 as  argument was not contained in the calling
	      program's address space.

       EINTR  A signal occurred before any requested event; see signal(7).

       EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

       ENOMEM There was no space to allocate file descriptor tables.

       The poll() system call was introduced in Linux 2.1.23.  On  older  ker‐
       nels  that  lack	 this  system call, the glibc (and the old Linux libc)
       poll() wrapper function provides emulation using select(2).

       The ppoll() system call was added  to  Linux  in	 kernel	 2.6.16.   The
       ppoll() library call was added in glibc 2.4.

       poll() conforms to POSIX.1-2001.	 ppoll() is Linux-specific.

       Some  implementations  define  the nonstandard constant INFTIM with the
       value -1 for use as a timeout for poll().  This constant	 is  not  pro‐
       vided in glibc.

       For  a  discussion  of what may happen if a file descriptor being moni‐
       tored by poll() is closed in another thread, see select(2).

   Linux notes
       The Linux ppoll() system call modifies its timeout_ts  argument.	  How‐
       ever,  the  glibc wrapper function hides this behavior by using a local
       variable for the timeout argument that is passed to  the	 system	 call.
       Thus,  the  glibc ppoll() function does not modify its timeout_ts argu‐

       See the discussion of spurious readiness notifications under  the  BUGS
       section of select(2).

       restart_syscall(2), select(2), select_tut(2), time(7)

       This  page  is  part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2013-07-30			       POLL(2)

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